The invention relates to an improved design for a spray nozzle and application system for cold gas dynamic spraying of a metal, alloy, polymer, or mechanical mixtures thereof. The gas and particles are formed into a supersonic jet having a temperature below the fusing temperature of the powder material, the jet being directed against an article which is to be coated.
There are various features which characterize a typical cold spray system of the Prior Art as illustrated in U.S. Pat. No. 5,302,414. The nozzle has been typically made of two halves for ease of fabrication. Clamps or bolt and nuts are used at multiple locations along the length to clamp the two halves together and ensure leak tightness. Such multiple point clamping of the nozzle, which is heated by the high temperature gas flowing through the nozzle, results in warping of the nozzle halves. This causes gas leakage between the two halves. The nozzle is attached to a 3-5 mm thick washer and this washer is bolted onto the gun body. Bolting the washer onto the gun body provides metal to metal seal at the injection point. This arrangement again causes warping of the washer and uneven bolting pressure results in gas and powder leakage between the gun body and the nozzle at the mating point. The entire gun body, consisting of the inlet chamber, gas and powder injection ports, mixing chamber and the diffuser, is welded together to form a single monolithic gun body. Thorough cleaning of the gun almost impossible. Moreover damage to any small part, like the diffuser, necessitates the whole gun body to be replaced.
In general, a bulky and heavy electrical heater is used to heat the large volume processing gas. Typical designs used today require the gun to be mounted right onto the heater. This arrangement necessitates that, in order to scan the substrate surface to produce a coating, either one has to move the substrate or move the whole heater-and-gun assembly. In many occasions, moving the substrate is not possible. Moving the gun and the heater assembly requires a heavy duty robot or manipulator and also restricts freedom of movement of the spray beam. Thus, the flexibility of the spray operation is highly restricted in this arrangement.
The heater normally heats the gas to as high as 1300xc2x0 F. The electric heating element, used to heat the process gas, operates under high pressure and temperature environment. During the spraying of some materials such as aluminum, the powder particles get deposited inside the nozzle on the walls blocking the gas flow path. When the nozzle block happens, the gas flow is reduced or even stopped causing abnormal increase in the temperature and pressure of the heating element and the gun. Such sudden increase in temperature and pressure can damage the gun and the heater, and also affect the safety of the operator.
The Prior Art is limited in both nozzle design and system configuration By using the novel design of the present invention, coupled with the new system arrangement of the essential elements of the invention, a more flexible configuration is shown which overcomes the inherent limitations of the teachings of the Prior Art as well as permitting a wider range of applications, not permitted with the presently available systems.
The invention eliminates many of the inherent limitations of the Prior Art by redesigning the nozzle which minimizes warpage at operating temperatures and a leak-tight joint, yet is still made of two halves for ease of fabrication. This new design uses a tapered cylindrical nozzle, in contrast to the rectangular nozzle design of the Prior Art. The cylindrical nozzle is held in place by a cylindrical nozzle holder with a complementary internal taper to that of the external taper of the nozzle, holding the two halves of the nozzle in position and sealing the joint with a uniform application of pressure over the entire length of the nozzle. In light of the fact that the nozzle holder is larger than the nozzle, it remains cooler than the nozzle, which expands due to the hot gas passing internally therein, thereby additionally facilitating the leak-proof fit of the nozzle to the nozzle holder. The invention additionally capitalizes on a remote gas heating step, which permits the disassociation of the heater mechanism from that of the main body of the gun, thereby permitting more flexibility in the application of the spray gun and allowing applicability in deposition geometries which would have been physically precluded by the Prior Art.
It is an object of this invention to improve the fluid dynamics at the nozzle.
It is another object of this invention to heat the gas and/or gases at a location remote from the spray gun which permits greater flexibility in system design for application onto substrates.
It is yet another object of this invention to enhance and/or maintain the leak-tight characteristics of the nozzle over time due to the improved design of the nozzle.